A method for controlling the speed of a vehicle

ABSTRACT

A method is provided for controlling the speed of a vehicle that includes a drive train for driving the vehicle and a predetermined preferred speed set. The method includes determining whether or not a substantially constant speed of the vehicle is requested, if it is determined that a substantially constant speed is requested, determining whether the requested speed lies within or outside of the predetermined preferred speed set, and if it is determined that the requested speed lies outside of the predetermined speed set, automatically adjusting the speed of the vehicle to a vehicle speed that is within the predetermined speed set.

BACKGROUND AND SUMMARY

The present disclosure relates to a method for controlling the speed ofa vehicle.

Various types of vehicles of today include a drive train for driving thevehicle and a speed control arrangement for controlling the speed of thevehicle. As such, when an operator of the vehicle requests a certainspeed, for instance by actuating the accelerator pedal (or gas pedal),the speed control arrangement communicates with the drive train suchthat the drive train operates in a working condition in which therequested speed is obtained.

However, there is a risk that the requested speed is not preferred forthe vehicle. Purely by way of example, the requested speed may result inthat the drive train operates in a working condition that hasinappropriately high power losses in for instance a power supply and/ora transmission arrangement of the drive train. As another example, therequested speed may result in an undesired wear of components of thevehicle. In a further example, the requested speed may result ininappropriate vibrations of, and/or noise from, at least some componentsof the vehicle.

In an attempt to solve the above problems, U.S. Pat. No. 7,789,795proposes that the engine speed of a power supply is selected so as tocorrespond to an optimum power supply efficiency and that the ratio ofautomatic transmission arrangement is selected so as to correspond to arequested power demand.

However, although the '795 method may result in some operatingconditions of the drive line in which the power losses of the drivetrain are relatively low, the '795 method may also result in operatingconditions in which the power losses are relative large. An example ofan operating condition wherein the power losses in the drive line areabove a preferred value may be when the selected engine speed and therequested velocity of the vehicle result in a ratio of the automatictransmission arrangement which ratio is associated with relatively largepower losses.

In view of the above, it is desirable to obtain a vehicle control methodwherein a drive line of the vehicle, at least during some steady stateoperating conditions, is operated such that preferred operatingconditions are obtained.

As such, the present disclosure relates according to an aspect thereofto a method for controlling the speed of a vehicle. The vehiclecomprises a drive train for driving the vehicle and the vehicle alsocomprises a predetermined preferred speed set. The method comprises:

-   -   determining whether or not a substantially constant speed of the        vehicle is requested;    -   if it is determined that a substantially constant speed is        requested, determining whether the requested speed lies within        or outside of the predetermined preferred speed set, and    -   if it is determined that the requested speed lies outside of the        predetermined speed set, automatically adjusting the speed of        the vehicle to a vehicle speed that is within the predetermined        speed set.

A method according to the above implies that the vehicle will obtain asubstantially constant speed that lies within the predetermined speedset and this in turn implies that the vehicle will be operated in apreferred operating condition when travelling at a substantiallyconstant speed.

As used herein, the expression “speed set” relates to at least onespeed. However, it should be noted that the expression “speed set” mayinclude a plurality of individual speeds. Moreover, in addition to, orinstead of, comprising at least one speed, the speed set may compriseone or more speed ranges. As such, the expression “speed set”encompasses any combination of at least one speed and/or at least onespeed range.

Optionally, a power loss of the drive train is equal to or below apredetermined power loss within the predetermined preferred speed set.

Optionally, the predetermined preferred speed set comprises a range ofpreferred speeds, preferably the predetermined preferred speed setcomprises a plurality of ranges of preferred speeds.

Optionally, the drive train comprises a power supply and an automatictransmission arrangement which comprises a power split assembly which inturn comprises a step ratio power flow path and a infinitely variableratio power flow path, at least one value in the predetermined preferredspeed set corresponding to at least one condition of the transmissionarrangement in which at least a predetermined portion of the power flowsthrough the step ratio power flow path.

Optionally, the predetermined portion of the power flow through the stepratio power flow path is at least 80%, preferably at least 90%, of thepower flow through the power split assembly.

Optionally, the vehicle comprises a plurality of predetermined preferredspeed sets, the method further comprising:

-   -   determining whether the vehicle speed control system requested        an acceleration or a deceleration of the vehicle prior to the        substantially constant speed request.

Optionally, if an acceleration of the vehicle was requested prior to thesubstantially constant speed request and if it is determined that therequested speed lies outside the predetermined preferred speed set:

-   -   increasing the speed of the vehicle until it reaches a speed        within the preferred speed set.

Optionally, if an deceleration of the vehicle was requested prior to thesubstantially constant speed request and if it is determined that therequested speed lies outside of the predetermined preferred speed set:

-   -   decreasing the speed of the vehicle until it reaches a speed        within the preferred speed set.

Optionally, the vehicle further comprises a driveline characteristicsvalue input means, the method further comprising:

-   -   determining the driveline characteristics value from the        driveline characteristics value input means, and    -   electing the range of preferred speeds in accordance with the        driveline characteristics value.

Optionally, the vehicle further comprises a manual speed control means,the method further comprising:

-   -   determining whether or not a substantially constant speed of the        vehicle is requested by the manual speed control means.

Optionally, the vehicle further comprises an automatic speed controlmeans, wherein the step of automatically adjusting the speed of thevehicle comprises a step of operating the automatic speed control meansin order to modify the speed of the vehicle.

A second aspect of the present disclosure relates to a computer programproduct comprising a computer program containing computer program codeexecutable in a computer or a processor to implement the steps of amethod according to any of the preceding claims, the product beingstored on a computer-readable medium or a carrier wave.

A third aspect of the present disclosure relates to an electroniccontrol unit comprising a computer program product according to thesecond aspect of the present disclosure and arranged to execute a methodfor controlling the speed of a vehicle according to the first aspect ofthe present disclosure.

A fourth aspect of the present disclosure relates to a vehiclecomprising a drive train adapted to drive the vehicle, the drive traincomprising a power supply and a transmission arrangement. The vehiclecomprises an electronic control unit according to the third aspect ofthe present invention.

Optionally, the vehicle further comprises the driveline characteristicsvalue input means.

Further advantages and advantageous features of the disclosure aredisclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detaileddescription of embodiments of the invention cited as examples.

In the drawings:

FIG. 1 illustrates a vehicle;

FIG. 2 illustrates a drive train for a vehicle;

FIG. 3 illustrates a diagram of a characteristics of a vehicle as afunction of the speed of the vehicle;

FIG. 4 illustrates a diagram of the velocity of a vehicle as a functionof time;

FIG. 5 illustrates another diagram of the velocity of a vehicle as afunction of time;

FIG. 6 schematically illustrates a transmission arrangement of avehicle;

FIG. 7 illustrates a torque curve of an internal combustion engine, and

FIG. 8 illustrates a diagram of a power loss of a vehicle as a functionof the speed of the vehicle.

DETAILED DESCRIPTION

The invention will below be described for a vehicle in the form of awheel loader 10 such as the one illustrated in FIG. 1. The wheel loader10 should be seen as an example of a vehicle in which the speed controlmethod of the present invention may be implemented. However, the speedcontrol method of the present invention may be implemented in aplurality of different types of vehicles. Purely by way of example, thespeed control method could be implemented in a truck, a lorry, atractor, a car, a bus or any type of work machine.

FIG. 2 schematically illustrates a drive train 12 of a vehicle. As maybe gleaned from FIG. 2, the drive train 12 comprises a power supply 14.Purely by way of example, the power supply 14 may be an internalcombustion engine, such as a diesel engine or a gasoline engine, or anelectric motor. As another example, the power supply 14 may be a socalled hybrid engine comprising an internal combustion engine as well asan electric motor (not shown).

The FIG. 2 drive train 12 may further comprise a transmissionarrangement 16. Purely by way of example, the transmission arrangementmay be a manual transmission arrangement and/or an automatictransmission arrangement. It should also be noted that inimplementations of the drive train 12, for instance when the powersupply 14 is an electric motor, the transmission arrangement may beomitted.

The drive train 12 may further comprise a drive shaft 18 adapted toconnect the power supply 14, possibly via a transmission arrangement 16,to one or more wheels of the drive train 12. In the FIG. 2implementation, the drive shaft 18 connects the transmission arrangement16 to the back wheels 20 of the drive train 12. However, in otherimplementations of the drive train 12, the drive shaft 18 may instead,or in addition, connect the transmission arrangement 16 to the forwardwheels 22.

The drive train 12 preferably also comprises an ECU (electronic controlunit) 24 and a speed request means, or speed requester, 26 forrequesting a desired speed of the vehicle 10 hosting the FIG. 2 drivetrain 12. The speed request means 26 may for instance comprise amanually operated actuator, such as an accelerator pedal or a gas pedal.

In other implementations of the drive train 12, the speed request means26 may be operated automatically. For instance, the speed request means26 may be operated so as to follow a target object (such as a targetvehicle) or the speed request means 26 may be implemented to request aspeed of the vehicle in accordance with a predetermined schedule.

As another example, the speed request means 26 may comprise means forreceiving information as regards a required speed and to transmit such arequest to selected components of the drive train 12. Purely by way ofexample, the speed request means 26 may be adapted to receiveinformation that is transmitted via a carrier wave and/or one or morecables (not shown in FIG. 2).

In the FIG. 2 implementation of the drive train 12, the speed requestmeans 26 is in communication with the ECU 24 which in turn is incommunication to the power supply 14 and/or the transmission arrangement16. However, in other implementations of the drive train 12, the speedrequest means 26 may be directly or indirectly in communication with thepower supply 14 and/or the transmission arrangement 16 withoutcommunicating with the ECU 24.

FIG. 2 further illustrates that the vehicle 10 may comprise aspeedometer 28 adapted to determine the speed of the vehicle 10. Purelyby way of example, the speedometer 28 may be adapted to measure therotational speed of a least one wheel 20, 22 of the vehicle 10. However,in other implementations, the speedometer 28 may be adapted to determinethe speed of the vehicle by using other means, such as a GPS system orthe like.

FIG. 3 is a diagram of a vehicle characteristics value as a function ofthe speed of the vehicle. The vehicle characteristics value isindicative of a characteristic of the vehicle. Purely by way of example,the vehicle characteristics value may be indicative of the wear of oneor more components of the vehicle. As another non-limiting example, thevehicle characteristics value may be related to the amount of vibrationsin one or more components of the vehicle. As a further non-limitingexample, the vehicle characteristics value may be indicative of thenoise level within and/or outside of the vehicle 10.

In the below examples, the vehicle characteristics value is indicativeof the power loss of the drive train 12. However, it should be notedthat the below description is equally applicable for any other type ofvehicle characteristics value.

FIG. 3 illustrates that a vehicle characteristics value 30, which in thepresent example is indicative of the power loss of the drive train 12,varies with the speed of the vehicle 10. In FIG. 3 a low value of thevehicle characteristics value is indicative of low losses of the drivetrain 12. As such, a low vehicle characteristics value is generallydesired. However, in other implementations, a high vehiclecharacteristics value may be indicative of a beneficial operatingcondition of the drive train 12 and/or the vehicle 10.

FIG. 3 further illustrates that for certain speeds, the vehiclecharacteristics value 30 is below a threshold value 32. The thresholdvalue 32 may be indicative of a largest preferred power loss of thedrive train 12 and/i or the vehicle 10 such that power losses below thethreshold value 32 may be acceptable and/or preferred. As such, in FIG.3, a predetermined preferred speed set 34 may be established thatcomprises three speed ranges 36, 38 and 40 as well as a single speedvalue 42. However, it should be noted that speed ranges and/orindividual values of a speed in a predetermined preferred speed set neednot necessarily be determined by using a constant threshold value 32.Quite the contrary, in other embodiments of the present invention,different threshold values may be used for different velocities (seee.g. FIG. 8).

Once a predetermined preferred speed set 34 has been established for avehicle 10, the speed set 34 is preferably stored in a readable mediumthat is accessible to the vehicle 10. Purely by way of example, thepredetermined preferred speed set 34 may be stored in the ECU 24 of thevehicle 10.

FIG. 4 is a diagram of the speed V of a vehicle 10 as a function of timeT. FIG. 4 further illustrates two of the speed ranges 36, 38 of thespeed set 34 illustrated in FIG. 3. FIG. 4 further illustrates a firsttime interval 44 during which an increase of the speed V of the vehicle10 is requested. During a second time interval 46, the speed controllingmethod of the present invention determines that a substantially constantspeed of the vehicle 10 is requested.

The step of determining whether or not a substantially constant speed ofthe vehicle 10 is requested may be implemented in a plurality of ways.Purely by way of example, the step may be implemented by determining,either constantly or at certain time instants, whether or not thedifference between a maximum speed and a minimum speed of the vehicle 10during an elapsed time exceeds a predetermined speed differencethreshold. The speed difference threshold may for instance be anabsolute value, e.g. expressed in km/h, or a relative value, e.g.expressed in a ratio of the average velocity of the vehicle 10.

As an example of the above implementation, the step of determiningwhether or not a substantially constant speed of the vehicle 10 isrequested may be determined by constantly, or at certain time instants,establishing whether or not the maximum speed difference exceeds 5 km/h(optionally, exceeds 5% of the average speed) for the last five seconds.If the maximum speed difference does not exceed the predetermined speeddifference threshold during the elapsed time, the above implementationconstant speed determination step establishes that a substantiallyconstant speed is requested.

As another option to implement the step of determining whether or not asubstantially constant speed of the vehicle 10 is requested, a measureindicative of the variance of the speed of the vehicle during apredetermined time is determined constantly or at certain time instants.If the above measure indicative of the speed variance is below apredetermined threshold value, the above implementation of the constantspeed determination step establishes that a substantially constant speedis requested.

It should be noted that the step of determining whether or not asubstantially constant speed of the vehicle 10 is requested may beimplemented in other ways in other embodiments of the speed controlmethod of the present invention.

Regardless of how it has been determined that a substantially constantspeed is requested, the control method of the present invention thendetermines whether the requested speed lies within or outside of thepredetermined preferred speed set 34. In embodiments of the inventivemethod, an average speed during a certain elapsed time may be used as ameasure of the requested speed.

As may be gleaned from FIG. 4, the requested, and substantiallyconstant, speed during the second time interval 46 is above the firstspeed range 36 and below the second speed range 38. As such, the controlmethod of the present invention determines that the requested speed liesoutside of the predetermined preferred speed set 34.

The speed control method of the present invention subsequentlyautomatically adjusts the speed of the vehicle 10 to a vehicle speedthat is within the predetermined speed set 34. Such an automaticadjustment may preferably be performed by the ECU 24, see FIG. 2. Assuch, during a third time interval 48 in FIG. 4, the speed of thevehicle 10 has been increased to the lowermost speed of the second speedrange 38.

If the substantially constant speed requested is located between twosubsets (each subset comprising a individual speed and/or a speed range)of the predetermined speed set 34, the speed control method preferablycomprises a step for selecting to which one of the two subsets thevelocity of the vehicle 10 should be adjusted.

The above selection may be implemented in a plurality of ways. Purely byway of example, the selection may be implemented such that the vehicle's10 speed is adjusted such that the adjusted speed of the vehicle is thespeed within the said predetermined speed set which is closest to thesubstantially constant requested speed.

As another non-limiting example, the above selection may be implementedsuch that the vehicle's speed is reduced until the vehicle's speed iswithin said predetermined speed set.

As a further non-limiting example, the above selection may beimplemented such that the vehicle's speed is increased until thevehicle's speed is within said predetermined speed set.

However, the time intervals 44, 46, 48 of FIG. 4 illustrate a preferredimplementation of the above selection. As has previously been discussed,the speed control method of the present invention has determined that asubstantially constant speed is requested during the second timeinterval 46. Once it has been established that a substantially constantspeed is requested, the preferred implementation of the above selectiondetermines whether an acceleration or a deceleration of the vehicle wasrequested prior to the substantially constant speed request.

In the example illustrated in FIG. 4, an acceleration of the vehicle 10was requested during the first time interval 44. The preferredimplementation of the selection increases the speed of the vehicle 10until it reaches a speed within the preferred speed set. In the exampleillustrated in FIG. 4, the speed is increased until it reaches thesecond speed range 38.

If the preferred implementation of the above selection insteaddetermines that a deceleration of the vehicle 10 was requested prior tothe substantially constant speed request, the preferred implementationinstead decreases the speed of the vehicle 10 until it reaches a speedwithin the preferred speed set.

The above scenario is illustrated in the fourth 50, fifth 52 and sixth54 time intervals in FIG. 4. A substantially constant speed is requestedduring the fifth time interval 54 and a deceleration of the vehicle 10was requested during the fourth time interval 50. Thus, the preferredimplementation of the selection reduces the speed of the vehicle at thefifth time interval 52 until it reaches the highest speed of the firstspeed range 36.

In an embodiment of the speed control method of the present invention, adriveline characteristics value is selected. Purely by way of example, adriveline characteristics value may be selected by actuating a drivelinecharacteristics value input means, or a driveline characteristics valueinputter, such as a driveline characteristics value actuator, of avehicle 10. An example of a driveline characteristics value input means29 is illustrated in FIG. 2. As may be gleaned from FIG. 2, thedriveline characteristics value input means 29 may preferably be incommunication with the ECU 24.

The driveline characteristics set may be a continuous or discrete rangeof data indicative of to what extent the driver and/or operator of thevehicle 10 may select a constant speed for the vehicle. Purely by way ofexample, the driveline characteristics set may comprise the values I, IIand III or the states Sport, Regular and Economy.

Each one of the above values of the driveline characteristics set may beassociated with a corresponding predetermined speed set. An example ofthree different predetermined speed set is illustrated in FIG. 5. InFIG. 5, the speed set comprising the widest first and second speedranges 36′, 38′ corresponds to the Sport state (or state I), the speedset comprising the first and second speed ranges 36″, 38″ of averagewidth corresponds to the Regular state (or state II) and the narrowestfirst and second speed ranges 36′″, 38′″ corresponds to the Economystate (or state III).

As may be gleaned from FIG. 5, the Sport state (or state I) allows adriver or operator of the vehicle to select a constant speed of thevehicle which speed is within relatively wide speed ranges. The widespeed ranges will result in a relatively high degree of freedom for thedriver/operator but may also imply that the vehicle may be driven at aspeed which is not necessarily close to the optimum vehicle speed, atleast from the vehicle characteristics value point of view.

On the other hand, the Economy state (or state III) with relativelynarrow speed ranges implies that an appropriate value, at least from thevehicle characteristics value point of view, of the constant speed maybe selected. However, the narrow speed ranges of the Economy state (orstate III) imply that the driver/operator of the vehicle will experiencea limited degree of freedom when operating the vehicle 10.

The Regular state (or state II) is a compromise between state I andstate III.

Other embodiments of the speed control method of the present inventionmay of course include more or fewer discrete values than what has beendiscussed hereinabove.

Furthermore, in embodiments of the speed control method comprising adriveline characteristics which in turn comprises a continuous datarange, the width of the speed ranges may be continuously adjusted inrelation to a selected value within the continuous data range.

In FIG. 5, illustrating an embodiment comprising a drivelinecharacteristics set with three values I, II and III (or Sport, Regularand Economy states), the Regular state has been selected. During a firsttime interval 56 in FIG. 5, an acceleration of the vehicle 10 isrequested and during a second time interval 58, a substantially constantspeed is requested. The speed control method then automaticallyincreases the speed of the vehicle 10 until the speed reaches the secondspeed range 38″associated with the Regular state, see the third timeinterval 60 in FIG. 5.

As has been intimated hereinabove, the vehicle characteristics value maybe indicative of a power loss of the drive train 12.

FIG. 6 illustrates an embodiment of a drive train 12 comprising a powersupply 14 and a transmission arrangement 16. The FIG. 6 transmissionarrangement 16 comprises a power split assembly. The power splitassembly comprises a step ratio power flow path 62 and an infinitelyvariable ratio power flow path 64. The step ratio power flow path 62 isadapted to provide at least one, but preferably a plurality of, fixedgear ratio whereas the infinitely variable ratio power flow path 64 isadapted to provide a continuous gear ratio.

Purely by way of example, the step ratio power flow path 62 may comprisea plurality of gear wheels. The step ratio power flow path 62 mayfurther comprise means, e.g. a selector, for selecting which ones of twoor more wheels that should be arranged to mesh in order to obtain arequired fixed gear ratio.

Moreover, and again purely by way of example, the infinitely variableratio power flow path 64 may comprise a hydraulic transmission and/or abelt-driven conical-pulley transmission.

As such, the step ratio power flow path 62 and the infinitely variableratio power flow path 64 may be operated so that the together provide adesired gear ratio. To this end, the transmission arrangement 16preferably comprises a torque split arrangement 66 adapted toselectively distribute a torque provided from the power supply 14 to thestep ratio power flow path 62 and the infinitely variable ratio powerflow path 64, respectively.

Traditionally, the infinitely variable ratio power flow path 64 isgenerally associated with larger power losses than the step ratio powerflow path 62. As such, a preferred embodiment of the speed controlmethod of the present invention may include that at least one value inthe predetermined preferred speed set corresponds to at least onecondition of the transmission arrangement 16 in which at least apredetermined portion of the power flows through the step ratio powerflow path 62. As used herein, the expression “portion of the power” isintended to relate to the power transmitted to the step ratio power flowpath 62 divided by the power delivered from the power supply 14 to thetransmission arrangement 16.

Preferably, the predetermined portion of the power flow through the stepratio power flow path 62 is at least 80%, preferably at least 90%, ofthe power flow through the power split assembly.

The above preferred conditions of the transmission arrangement 16 maypreferably be combined with preferred operating conditions of the powersupply 14 in order to obtain at least a portion of a preferred speedset. To this end, reference is made to FIG. 7 illustrating a torquecurve 68 of a power supply 14 which in the FIG. 7 example is an internalcombustion engine. At least one torque area 70′ below the torque curve68 is determined, theoretically and/or by testing, to be associated withappropriately low losses in, i.e. with appropriately high efficiency of,the power supply 14.

As may be gleaned from FIG. 7, the torque area 70′ is associated with acertain engine speed range and/or a certain torque range. The rangesthus determined may subsequently be combined with the preferredcondition or conditions of the transmission arrangement 16 which havebeen discussed hereinabove in order to arrive at a predeterminedpreferred speed set.

In the implementation illustrated in FIG. 7, two preferred torque areas70′, 70″ are determined each one of which being associated with anappropriately high efficiency of the power supply 14. It should be notedthat the actual efficiency of the different torque areas need not be thesame. Moreover, in other implementations of the above step ofdetermining a preferred engine speed range, one, two, three or morepreferred torque areas (not shown in FIG. 7) may be determined for apower supply 14.

Instead of or in addition to, the above combination of preferredcondition or conditions of the transmission arrangement 16 and preferredengine speeds and/or torques, a predetermined preferred speed set may bedetermined, theoretically and/or by testing, by determining the powerloss of the assembly of the power supply 14 and the transmissionarrangement 16. FIG. 8 illustrates the power loss of the above assemblyas a function of the velocity of a vehicle hosting the assembly. FIG. 8also illustrates that certain speed ranges have been identified whichcorrespond to velocities that are equal to or below preferred powerlosses. FIG. 8 further illustrates that the magnitude of the preferredpower loss may vary with the speed of the vehicle 10.

It is to be understood that the present invention is not limited to theembodiments described above and illustrated in the drawings. As such,the skilled person will recognize that many changes and modificationsmay be made within the scope of the appended claims.

1. A method for controlling the speed of a vehicle (10), said vehicle(10) comprising a drive train (12) for driving said vehicle (10), saidvehicle (10) comprising a predetermined preferred speed set (34), saidmethod comprising: determining whether or not a substantially constantspeed of the vehicle (10) is requested; if it is determined that asubstantially constant speed is requested, determining whether therequested speed lies within or outside of said predetermined preferredspeed set (34), and if it is determined that the requested speed liesoutside of said predetermined speed set (34), automatically adjustingthe speed of the vehicle to a vehicle speed that is within saidpredetermined speed set (34).
 2. The method according to claim 1,wherein a power loss of said drive train (12) is equal to or below apredetermined power loss within said predetermined preferred speed set.3. The method according to claim 1 or 2, wherein said predeterminedpreferred speed set comprises a range (36, 38, 40; 72, 74, 76, 78) ofpreferred speeds, preferably said predetermined preferred speed setcomprises a plurality of ranges of preferred speeds.
 4. The methodaccording to any one of the preceding claims, wherein said drive train(12) comprises a power supply (14) and an automatic transmissionarrangement (16) which comprises a power split assembly which in turncomprises a step ratio power flow path (62) and a infinitely variableratio power flow path (64), at least one value in said predeterminedpreferred speed set corresponding to at least one condition of saidtransmission arrangement in which at least a predetermined portion ofthe power flows through said step ratio power flow path (62).
 5. Themethod according to claim 4, wherein said predetermined portion of saidpower flow through said step ratio power flow path (62) is at least 80%,preferably at least 90%, of the power flow through said power splitassembly.
 6. The method according to any one of the preceding claims,wherein said vehicle (10) comprises a plurality of predeterminedpreferred speed sets, said method further comprising: determiningwhether said vehicle speed control system requested an acceleration or adeceleration of the vehicle (10) prior to the substantially constantspeed request.
 7. The method according to claim 6, wherein, if anacceleration of the vehicle (10) was requested prior to thesubstantially constant speed request and if it is determined that therequested speed lies outside said predetermined preferred speed set:increasing the speed of the vehicle (10) until it reaches a speed withinsaid preferred speed set.
 8. The method according to claim 6 or 7,wherein, if an deceleration of the vehicle (10) was requested prior tothe substantially constant speed request and if it is determined thatthe requested speed lies outside of said predetermined preferred speedset: decreasing the speed of the vehicle (10) until it reaches a speedwithin said preferred speed set.
 9. The method according to any ofclaims 2 to 8, wherein said vehicle further comprises a drivelinecharacteristics value input means (29), said method further comprising:determining said driveline characteristics value from said drivelinecharacteristics value input means (29), and selecting said range ofpreferred speeds in accordance with said driveline characteristicsvalue.
 10. The method according to any one of the preceding claims,wherein said vehicle further comprises a manual speed control means(26), said method further comprising: determining whether or not asubstantially constant speed of the vehicle is requested by said manualspeed control means (26).
 11. The method according to any one of thepreceding claims, wherein said vehicle further comprises an automaticspeed control means, wherein the step of automatically adjusting thespeed of the vehicle comprises a step of operating said automatic speedcontrol means in order to modify the speed of the vehicle.
 12. Computerprogram product, characterized in that it comprises a computer programcontaining computer program code executable in a computer or a processorto implement the steps of a method according to any of the precedingclaims, said product being stored on a computer-readable medium or acarrier wave.
 13. Electronic control unit (24), characterized in that itcomprises a computer program product according to claim 12 and arrangedto execute a method for controlling the speed of a vehicle according toany one of claims 1-11.
 14. A vehicle (10), comprising a drive train(12) adapted to drive said vehicle (10), said drive train (12)comprising a power supply (14) and a transmission arrangement (16),characterized in that said vehicle (10) comprises an electronic controlunit (10) according to claim
 13. 15. The vehicle (10) according to claim14, when dependent on claim 9, wherein said vehicle (10) furthercomprises said driveline characteristics value input means (29).
 16. Amethod for controlling the speed of a vehicle, said vehicle comprising adrive train for driving said vehicle, said vehicle comprising apredetermined preferred speed set, said method comprising: determiningwhether or not a substantially constant speed of the vehicle isrequested; if it is determined that a substantially constant speed isrequested, determining whether the requested speed lies within oroutside of said predetermined preferred speed set, and if it isdetermined that the requested speed lies outside of said predeterminedspeed set, automatically adjusting the speed of the vehicle to a vehiclespeed that is within said predetermined speed set.
 17. The methodaccording to claim 16, wherein a power loss of said drive train is equalto or below a predetermined power loss within said predeterminedpreferred speed set.
 18. The method according to claim 16 or 17, whereinsaid predetermined preferred speed set comprises a range of preferredspeeds, preferably said predetermined preferred speed set comprises aplurality of ranges of preferred speeds.
 19. The method according to anyone of claims 16-18, wherein said drive train comprises a power supplyand an automatic transmission arrangement which comprises a power splitassembly which in turn comprises a step ratio power flow path and ainfinitely variable ratio power flow path, at least one value in saidpredetermined preferred speed set corresponding to at least onecondition of said transmission arrangement in which at least apredetermined portion of the power flows through said step ratio powerflow path.
 20. The method according to claim 19, wherein saidpredetermined portion of said power flow through said step ratio powerflow path is at least 80%, preferably at least 90%, of the power flowthrough said power split assembly.
 21. The method according to any oneof the preceding claims, wherein said vehicle comprises a plurality ofpredetermined preferred speed sets, said method further comprising:determining whether said vehicle speed control system requested anacceleration or a deceleration of the vehicle prior to the substantiallyconstant speed request.
 22. The method according to claim 21, wherein,if an acceleration of the vehicle was requested prior to thesubstantially constant speed request and if it is determined that therequested speed lies outside said predetermined preferred speed set:increasing the speed of the vehicle until it reaches a speed within saidpreferred speed set.
 23. The method according to claim 21 or 22,wherein, if an deceleration of the vehicle was requested prior to thesubstantially constant speed request and if it is determined that therequested speed lies outside of said predetermined preferred speed set:decreasing the speed of the vehicle until it reaches a speed within saidpreferred speed set.
 24. The method according to any of claims 17 to 23,wherein said vehicle further comprises a driveline characteristics valueinputter, said method further comprising: determining said drivelinecharacteristics value from said driveline characteristics valueinputter, and selecting said range of preferred speeds in accordancewith said driveline characteristics value.
 25. The method according toany of claims 17 to 24, wherein said vehicle further comprises a manualspeed controller, said method further comprising: determining whether ornot a substantially constant speed of the vehicle is requested by saidmanual speed controller.
 26. The method according to any of claims 17 to24, wherein said vehicle further comprises an automatic speedcontroller, wherein the step of automatically adjusting the speed of thevehicle comprises a step of operating said automatic speed controller inorder to modify the speed of the vehicle.
 27. Computer program product,comprising a computer program containing computer program codeexecutable in a computer or a processor to implement the steps of amethod according to any of the preceding claims, said product beingstored on a computer-readable medium or a carrier wave.
 28. Electroniccontrol unit, comprising a computer program product according to claim27 and arranged to execute a method for controlling the speed of avehicle according to any one of claims 16-26.
 29. A vehicle, comprisinga drive train adapted to drive said vehicle, said drive train comprisinga power supply and a transmission arrangement, wherein said vehicle (10)comprises an electronic control unit according to claim
 28. 30. Thevehicle according to claim 29, when dependent on claim 24, wherein saidvehicle further comprises said driveline characteristics value inputter.